The present invention relates to the general field of undersea pipes for transporting fluids by providing a bottom-to-surface connection for transferring hydrocarbons, e.g. oil and gas, coming from undersea production wells. More particularly, the present invention relates to the general field of undersea transport pipes, either of the rigid pipe type known as steel catenary risers (SCRs), or else of the flexible pipe type.
Undersea pipes for transporting fluids are commonly used in the offshore production of hydrocarbons. Thus, in an offshore production field there are generally a plurality of wells in operation, which wells may be spaced apart from one another by several kilometers, or even tens of kilometers. The fluids coming from the various wells need to be collected by pipes laid on the sea bottom and they need to be transferred by bottom-to-surface connection pipes from an undersea pipe resting on the sea bottom to a surface installation that collects them, e.g. on a ship or at a collection point situated on shore.
The invention relates more particularly to pipes that are fitted with floats or buoys that are immersed along at least a portion of the pipe in order to provide tensioning between the bottom and the surface so as to exert permanent tension on the pipe for holding the bottom-to-surface connection pipe in position, possibly in a vertical position.
More particularly, use is made of buoys that surround the pipe coaxially. Typically, a buoy module is generally cylindrical in shape, being hollow or solid, and being constituted by one or more portions, in particular two half-buoys, that, once they have been assembled together, form a cylindrical buoy surrounding the pipe coaxially. Still more particularly, sets of buoys are used that are arranged in a string along at least a portion of the pipe.
Generally, the buoys used for undersea fluid transport pipes in the offshore production of hydrocarbons are made of foam, e.g. of syntactic polypropylene (PP) or polyurethane (PU).
Since those undersea installations and equipment remain under water at great depths for long durations (20 years and more), installed buoys must also satisfy high mechanical stress requirements in order to withstand pressure at the sea bottom. Specifically, since the pressure of water is substantially 0.1 megapascals (MPa), i.e. about 1 bar, for a depth of 10 meters (m), the pressure that undersea pipes need to withstand is then about 10 MPa, i.e. about 100 bars, for a depth of 1000 m, and about 30 MPa, i.e. about 300 bars, for a depth of 3000 m.
Unfortunately, experience shows that such buoys deteriorate over time. Furthermore, deterioration of an underwater buoy leads to a reduction in its buoyancy, and thus in the thrust exerted by the buoy, which may be associated with deterioration of the buoyancy material constituting the buoy and/or with a change in the volume of the buoy, resulting from the pressure to which the buoy is subjected, in particular at great depth. For buoys based on syntactic foam, this deterioration of the buoy may be due to water infiltrating into the buoy, leading to hydrolysis of the glass beads contained in the buoy.
Optical fiber deformation measurement systems are known, e.g. for measuring curvature deformation or length deformation. With a Bragg grating fiber, a light beam is injected into the fiber, and a wave is reflected by each Bragg grating, with the wavelength of the reflected wave depending on the pitch of the grating and on the refractive index of the core of the fiber. With gratings having different pitches, it is possible to distinguish between the reflected waves. Because variation in stress and temperature affects both the refractive index and the pitch of a grating, which leads to a shift in the reflected wavelength, such a Bragg grating fiber is associated with a Raman effect type fiber for decorrelating the variation in the signal due to deformation of the fiber from the variation in the signal due to variation in temperature. For a Brillouin backscatter fiber, a light beam is injected into the fiber, with the backscattered wave being subjected to a frequency shift that depends on variation in temperature and on deformation. It is thus necessary also to associate such a fiber with a Raman effect type fiber (which measures temperature variation only), in order to decorrelate signal variation due to deformation of the fiber from signal variation due to variation in temperature. Using a laser traveling along the fiber, optical fiber deformation measurement technology serves to obtain the extent to which the fiber has lengthened and thus the deformation of the material that is in contact with the fiber. An optical fiber can measure a microdeformation and the microdeformation can be identified using Bragg gratings or using Brillouin backscattering. For a Bragg grating extension meter, when the optical fiber is subjected to deformation, the pitch of the Bragg grating (microstructure etched in the core of the fiber) changes so the way it reflects light is modified.
The object of the present invention is to provide a method for monitoring and evaluating in real time the thrust from buoys providing buoyancy to bottom-to-surface connection undersea pipes over their lifetime in order to be able to change them, should that be necessary.
US 2016/161350 describes a method of determining stresses in a support structure, specifically a pipe, on which an optical fiber is wound helically. Deformation of the fiber is measured in order to study the stresses on the pipe, but not the consequences of those stresses on potential degradation of the thrust properties of a buoy mounted on the pipe.
In WO 2009/158630, an optical fiber is used to measure the acoustic emission characteristics that result from a fluid leak in the pipe and to estimate the diameter of the orifice of the leak and its flow rate.
US 2014/338918 describes means for controlling and adjusting the buoyancy of a buoyancy chamber as a function of vertical tension variations applied on the borehole pipe in a well bored in the bottom of the sea. The buoyancy of the float is varied deliberately by adjusting the quantities of gas or of fluid contained in the float by means of ballasting and deballasting operations. For this purpose, in an implementation, a signal is measured from a load sensor arranged at an abutment against which the thrust from the float is applied (see paragraph 40, page 4, left-hand column). Nevertheless, there is no description nor suggestion of measuring the deformation of an optical fiber applied in or on an abutment part that is elastically deformable in compression in order to monitor variation in the thrust exerted by a buoy on a pipe on which it is applied, for the purpose of monitoring the state of the buoy.
WO 2014/016784 describes a method of monitoring the buoyancy of a float exerting traction on a pipe. It envisages (see paragraph 33) using an optical fiber as a strain detector, but gives no detail as to how it might be used.
US 2011/292384 describes detecting stress in the coating of a flexible pipe by using an optical fiber.
US 2016/084065, WO 2013/098546, and US 2004/035216 describe sensors for measuring various parameters including tensions and vibration by using an optical fiber applied on a riser, but without mentioning nor suggesting measuring variation in thrust from a buoy mounted on said pipe.
The present invention provides a method of monitoring variation over time in the thrust exerted by at least one buoy mounted on an undersea bottom-to-surface connection pipe and exerting traction on said pipe, the method being characterized in that the following steps are performed:
1) measuring the deformation of at least one optical fiber by measuring variation of an optical signal in the optical fiber relative to a reference value for the optical signal, said optical fiber extending at least in part in a direction having a component parallel to the direction of the thrust force exerted by said buoy on said pipe, said optical fiber being securely applied against the surface, preferably by adhesive, or preferably being securely embedded in the bulk of a material constituting at least an abutment part secured to said pipe or to the buoy and against which said buoy exerts said thrust, said abutment part being an elastically deformable intermediate abutment part arranged between i) a portion of the buoy and ii) a rigid retaining part secured to said pipe, the thrust exerted by said buoy comprising said intermediate abutment part; and
2) determining a variation in said thrust exerted by said buoy as a function of said variation of the optical signal as measured in step 1) relative to a value for the optical signal corresponding to a maximum initial thrust from the buoy.
In particular, said reference value of the optical signal is the initial value measured when the pipe has just been laid at sea, since it is at that moment when buoyancy thrust is at a maximum. All other measurements are compared with this initial state.
In practice, and by way of illustration, it may be found necessary to replace a buoy if its thrust decreases by 20% (thus if there is a corresponding determined variation in the measurement of the optical signal).
It can be understood that said optical fiber is connected to an optical fiber that may be integrated in an umbilical for conveying the optical signal, preferably along the pipe, and in particular for going up to the surface and down to the bottom of the sea.
Advantageously, a calibration step may be performed in a workshop on the basis of fiber deformation measurements performed in correlation with known variations of thrust values exerted by said buoy including a said fiber, said thrust possibly being measured by mechanical means (springs) or electronic means (strain gauges).
More particularly, in step 1), the deformation of at least one optical fiber is measured by using at least Brillouin backscattering or a Bragg grating optical fiber, by measuring variation in the frequency of the backscattered wave or respectively variation in the wavelength of the reflected wave from the optical signal reflected in the optical fiber compared with a reference value for the optical signal of the backscattered wave or respectively of the reflected wave, and preferably said optical fiber is associated with a fiber of the Raman effect type, in particular in the event of temperature variation in order to decorrelate the variation of the signal due to deformation of the fiber from the variation in the signal due to temperature variation.
In practice, this measurement of variation in buoyancy thrust is made possible since the materials used as materials for constituting buoys, namely synthetic materials based on PP or PU, preferably syntactic foam, or steel for pipes (themselves made of steel) may be subjected to deformation for the levels of thrust applied to the pipe by a buoy or a set of buoys, which are typically in the range 0.5 tonnes (t) to 5 t, and at these levels of thrust, the deformation of said materials constituting the buoys or their compressed abutment parts, or the deformation of the pipe in traction remains less than the maximum deformation acceptable for the optical fiber, typically around 1% in traction and 10% in compression. However, this problem can be avoided by using a fiber that is laid in non-rectilinear manner, e.g. as a helix, as described below. Thus the way the fiber is laid can be adapted to the material. If the material is rigid and lengthens less than the fiber, a fiber that is laid in a straight line can function. If the material is very deformable, and if it deforms more then the fiber, than the fiber cannot be laid in a straight line, but must be laid as a helix, for example.
If the optical fiber is applied on or in a support element constituted by an elastic material for said abutment part, said fiber is deformable in compression like the support elements in question.
It should be observed that the float is subjected in significant manner to swell, to current, and to movement of the float only if its depth is less than 50 m. Furthermore, and in any event, such dynamic stresses and the variations in their components along the thrust axis of the float are fast, so they can easily be filtered out and reveal only the deformation of the fiber and of the support that is caused by variation in the thrust from the buoy, which variation is much slower.
Preferably, said optical fiber is arranged in or at the surface of a said abutment part, which is an elastically deformable intermediate abutment part, made of elastomer material, said buoy being pressed against the initially compressed intermediate part, and a reduction in the thrust from the buoy over time is determined by elastic expansion of said intermediate part resulting from a decrease in the compression and a variation in the volume of the intermediate abutment part, said buoy being made of a buoyant material that is more rigid than the material of the intermediate abutment part and that is not subjected to elongation deformation in the event of degradation giving rise to a decrease in the thrust exerted by said buoy.
It can be understood that the rigid retaining part serves to hold said abutment part against movement in translation and thus to hold said buoy.
In this implementation, the buoy is pressed against the initially compressed intermediate part, and the intermediate part becomes less and less compressed over time when there is a reduction in thrust from the buoy over time. This implementation in which the optical fiber is adhesively bonded on the surface or is embedded in the bulk of an intermediate abutment part made of elastomer material is particularly advantageous since an elastomer material, even when of small size having thickness in the range 5 mm to 50 mm in the vertical direction, serves to make it possible to detect a reduction of thrust by elastic expansion of said intermediate part resulting from the reduction in compression and variation in the volume of the intermediate abutment part under circumstances in which a more rigid buoyant material, such as the material constituting the buoy, would not be subjected to variation in volume or the steel of the pipe would not be subjected to elongation deformation, thereby not enabling a variation in thrust to be detected. In particular, it can happen that the syntactic foam of the buoy loses buoyancy by degradation of the foam and of the glass beads that it contains, but without any variation in volume as a result of the foam and the glass being replaced by the surrounding water as a result of porosity.
More particularly, said buoy is of part cylindrical or pseudo-cylindrical shape arranged coaxially around the pipe.
This type of buoy is generally secured to the pipe by friction from clamping collars. The number of buoys and their positions along the pipe may vary depending on the installation and may depend on the method used for laying, on depth, and on the type of pipe that is to be laid.
More particularly, a said central cavity in the buoy having the pipe passing therethrough presents a wall portion of shape complementary to the shape of said rigid retaining part and suitable for blocking movement in translation of the buoy in the longitudinal direction of the pipe.
In a particular implementation, said rigid retaining part is constituted by a top peripheral collar made of steel surrounding said pipe and serving to hold in position said buoy arranged coaxially around the pipe and under said collar, said intermediate abutment part of annular shape being arranged coaxially against the underface of said rigid retaining part.
In this implementation, the assembly of said collar and said intermediate abutment part of annular shape arranged coaxially against the underface of said rigid retaining part can co-operate with a complementary hollow shape in an inside surface of a central cavity of said buoy that faces the outside surface of the pipe.
In this implementation, said buoy (or a plurality of buoys in series) may be slidably mounted around the pipe, being blocked against moving in translation by the collar that transmits thrust from the buoys to the pipe.
In another particular implementation, said rigid retaining part is constituted by a bottom peripheral collar made of steel surrounding said pipe and serving to hold in position said buoy arranged axially around the pipe and above said collar, said intermediate abutment part of annular shape being arranged coaxially against the underface of said rigid retaining part.
In this implementation, said buoy may be fastened to said pipe by a clamping element at its bottom end, which element is pressed against an underface of said compressed abutment part.
In another particular implementation, said rigid retaining part is constituted by a steel shoulder at the surface of the pipe, with the assembly of said shoulder and of said intermediate abutment part of annular shape arranged coaxially against the underface of said rigid retaining part co-operating with a complementary hollow shape in an inside surface of the central cavity of said buoy facing the outside surface of the pipe, said shoulder serving to hold in position said buoy arranged coaxially around the pipe and said shoulder.
An arbitrary distinction is made herein between a “shoulder” and a “collar” in that firstly a shoulder is a radial projection that extends over a greater distance in the longitudinal direction of the pipe or the coaxial buoy and secondly the buoy surrounds a shoulder completely.
Said shoulder may be a radial projection made integrally with the surface of the pipe or fitted onto the surface of the pipe using clamping elements, in particular in the form of two fitted semicylindrical retaining half-parts that are fastened one against the other and against the pipe by a belt or a clamp.
It can be understood that the buoy cannot move in translation or slide axially beyond the shoulder or collar, and the thrust force from the buoy is transmitted to the pipe via a bearing surface of the shoulder or collar, and possibly via the outside surface of the pipe facing a clamping element for clamping the buoy against the pipe.
More particularly, said collar or shoulder may extend in continuous or discontinuous manner circularly around the pipe with a thickness in the range 5 millimeters (mm) to 50 mm from the surface of the pipe.
Still more particularly, said collar or shoulder may be arranged in association with a forged junction part providing the welded junction between two unit pipe elements of said pipe.
In a variant implementation, the fiber extends helically and coaxially on the surface of or inside said support element. This implementation in which the optical fiber is arranged in a helix is preferable since a helical lay imparts curvature to a fiber enabling it to accept greater deformation of said support element and more particularly of the deformable part, and it makes it possible to use a single fiber. The deformation of the optical fiber in the axial direction of the thrust is reduced by a factor that corresponds to the cosine of the pitch angle of the helix.
In a variant implementation, the optical fiber extends sinusoidally or in undulations or as a zigzag around at least a portion of the circumference and preferably the entire circumference on the surface of or inside said support element.
It can be understood that the amplitude axis of the sinusoid or of the undulations corresponds to the axis of the buoy and of the pipe, and the axis of the period of the sinusoid is circular. This implementation using a sinusoidal arrangement of the optical fiber is also advantageous since the sinusoidal lay also imparts curvature to the fiber enabling it to accept greater deformation of said support element and more particularly of the deformable part, and it makes it possible to use a single fiber.
In a variant implementation, the optical fiber extends over a toroidal surface, preferably by winding the fiber on an imaginary torus or on a toroidal support of diameter smaller than the thickness of said support element, and preferably situated in said intermediate abutment.
In a variant embodiment, a plurality of optical fibers are used that extend in rectilinear manner in said coaxial direction of said support element.
More particularly, a plurality of rectilinear optical fibers are used that are arranged regularly around the circumference of said support element comprising said buoy, or preferably the surface of the pipe.
In this implementation, the plurality of fibers is advantageous for detecting deformation over the entire periphery or circumference of the pipe or buoy.
The present invention also provides a bottom-to-surface connection installation suitable for use in a method of the invention, the installation being characterized in that it comprises a bottom-to-surface connection undersea pipe fitted with at least one buoy exerting traction on said pipe, and at least one optical fiber suitable for measuring deformation of the pipe by measuring variation of the optical signal in the optical fiber relative to a reference value for the optical signal, said optical fiber extending at least in part in a direction having a component parallel to the direction of the thrust force exerted by said buoy on said pipe, said optical fiber being securely applied against the surface, preferably by adhesive or being securely embedded in the bulk of a material constituting at least an abutment part secured to said pipe or to the buoy and against which said buoy exerts said thrust, said abutment part being an elastically deformable intermediate abutment part arranged between i) a portion of the buoy and ii) a rigid retaining part secured to said pipe, the thrust exerted by said buoy compressing said intermediate abutment part; and                said optical fiber is connected to an optical fiber conveying the optical signal, in particular in an umbilical containing an optical fiber, preferably along the pipe.        
More particularly, said installation comprises a plurality of said buoys of part cylindrical or pseudo-cylindrical shape arranged one after another or spaced apart in a string coaxially around the pipe, and at least one optical fiber for measuring the respective thrusts from each of the buoys, preferably a single optical fiber for all of the buoys.
More particularly, the or each buoy is constituted by a plurality of buoy elements, preferably in the form of two buoy half-modules of semicylindrical shape suitable for being arranged facing each other so as to surround the pipe.
More particularly, at least one optical fiber is arranged in or at the surface of a said elastically deformable intermediate abutment part that is of annular shape and made of elastomer material, the thrust exerted by said buoy compressing said intermediate abutment part, said portion of said central cavity of the buoy presenting a shape that is complementary to the shape of the assembly of said abutment part and said rigid retaining part suitable for blocking movement of the buoy in translation along the longitudinal direction of the pipe.
More particularly, said buoy is applied against the initially compressed intermediate part, said buoy being made of a buoyant material that is more rigid than the material of said intermediate abutment part and that is not deformed in elongation in the event of degradation that gives rise to a decrease in the thrust exerted by said buoy.
Preferably, the optical fiber(s) of various support elements of a plurality of buoys in series or in a string is/are connected in series and to a common umbilical for transmitting optical signals to an optical signal transceiver and measuring device that is preferably at the surface.
In this embodiment, knowing the return time of the signal, and on the basis of the speed of light, it is possible to determine the position of a deformation and thus the fiber or fiber portion that has been deformed, and thus which buoy has been degraded.
In another embodiment, the optical fiber(s) of the various support elements of a plurality of buoys in series or in a string is/are not connected in series, being suitable for connecting to different umbilicals for transmitting optical signals.
Alternatively, if each buoy support element has its own fiber or its own fiber network not connected in series between the various buoys, it suffices to make connections in turn with an umbilical and the optical fiber(s) of each of the support elements.
Advantageously, the buoyancy material of the buoy may be a material that also presents thermal insulation properties, such as syntactic foam.